1. Field of the Invention
The present invention relates to a surface acoustic wave device including a surface acoustic wave element hermetically sealed in a package including a base member and a conductive cap member joined together by a sealing material.
2. Description of the Related Art
A conventional surface acoustic wave device includes, for example, as shown in FIG. 3, a concave base member 1 made of a ceramic such as alumina, and a surface acoustic wave element 2 bonded and fixed to the base member 1 by an adhesive (e.g. via die bonding). Certain electrodes of the surface acoustic wave element 2 and certain electrodes of the base member 1 are connected to each other through bonding wires 5, then the base member 1 and a cap member 3 are joined by a sealing material 7, such as a resin adhesive, a solder, or a glass, whereby the surface acoustic wave element 2 is hermetically sealed in a package defined by the base member 1 and the cap member 3. The surface acoustic wave element is provided with microscopic electrodes (not shown) defining an IDT on the surface thereof disposed across a gap of several xcexcm or several tens of xcexcm between each electrode.
In the surface acoustic wave device of this type, the surface acoustic wave element must be hermetically sealed so that moisture or the like is prevented from penetrating the package in order to prevent deterioration of the characteristics of the surface acoustic wave element.
Also, in this type of device, a conductive cap member is used as the cap member 3. The conductive cap member is made of a punched metallic plate or an insulating material such as a ceramic and is processed to be conductive by, for example, plating.
In this prior art method, the base member 1 and the cap member 3 are joined in a manner such that the sealing material 7 is formed to have a frame-shape by a method such as printing on the joining portion of either the base member 1 or the cap member 3, and the sealing material 7 is cured or melted.
However, in the above-described known surface acoustic wave device, a problem has been discovered in that, as a result of the sealing material being formed to have a frame-shape at the peripheries of the base member and the cap member, metallic particles, such as punched-out particles or scraps, adhered to the cap member when the cap member is punched or processed to be conductive, drop on the surface of the surface acoustic wave element while the base member and the cap member are joined, or in subsequent processes or while being used, thereby deteriorating the characteristics thereof.
Another problem has been discovered in that the required air-tightness cannot be sufficiently and reliably achieved because the length of paths of moisture from the outside to the inside of the package is the same as only the width of the sealing material, the passages being formed either through the joining interfaces between the sealing material and each of the members or through the sealing material itself, and because the joining interfaces are exposed inside of the internal space.
A further problem has been discovered in that inadequate joining, such as lack of adhesive strength or decreased air-tightness, occurs due to the lack of the sealing material for sealing gaps when the gaps are formed between the base member and the cap member because of particles adhered at the joining portion thereof or because scratches on the base member and the cap member.
To overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device in which metallic particles adhered to a conductive cap member are prevented from adhering to a surface acoustic wave element.
In order to achieve these and other advantages, a preferred embodiment of the present invention provides a surface acoustic wave device which includes a surface acoustic wave element and a package. The package has a base member and a conductive cap member which are joined together by a sealing material so that the surface acoustic wave element is hermetically sealed in the package. The sealing material is provided on the entire surface of the conductive cap member that is disposed opposite to the base member.
A material, such as a resin adhesive, a solder, a gold-tin (Auxe2x80x94Sn) alloy, or a glass, is preferably used as the sealing material. A resin adhesive is preferably used so as to facilitate coating and joining processes. In particular, an epoxy-based resin adhesive having high density is preferably used in order to obtain a high level of air-tightness and a sufficient adhesive strength.
With this configuration, the conductive cap member is coated in advance with the sealing material on the entire surface thereof that is disposed opposite to the base member, whereby metallic particles such as punched-out particles and scraps adhered to the conductive cap member in the manufacturing process are fixed to the sealing material. Therefore, the metallic particles are not adhered to the surface of the surface acoustic wave element when both members are joined, nor in subsequent processes or while being used, thereby avoiding deterioration of the characteristics or occurrence of deteriorated characteristics due to the metallic particles adhering to the surface acoustic wave element.
One of the joining interfaces of the sealing material which define penetrating paths of outside air (moisture, gases, etc.), which is the joining interface of the sealing material with the cap member, is not exposed inside the package, and the length of paths, that is, the joining interface of the sealing material with the base member and the length of paths through the sealing material itself are increased in size to be relatively large. As a result, the penetration of the outside air, which penetrates from the outside to the inside, is greatly reduced and air-tightness is greatly improved. The effect of prevention of the outside air from penetrating is particularly remarkable when using a resin adhesive through which the outside air penetrates more than through other sealing materials such as a solder and glass.
Inadequate joining due to lack of adhesive strength, decreased air-tightness, or other such conditions does not occur when gaps are produced between the base member and the cap member by foreign particles adhered at the joining portion or scratches formed on the base member or the cap member, because, in this case, the sealing material adhered (coated) toward the inside from the joining portion is supplied to the joining portion.
The surface acoustic wave device according to preferred embodiments of the present invention generally includes devices, such as a surface acoustic resonator, a surface acoustic filter, and surface acoustic delay line, which use surface acoustic waves. Each device is provided with at least one IDT disposed on a piezoelectric substrate thereof. The number of IDTs are determined according to the type and use of the device.
According to preferred embodiments of the present invention, a reliable surface acoustic wave device having superior electrical characteristics and resistance to natural phenomena is achieved.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.